Fluid Flow CONTINUITY EQUATION This equation tells us that the pressure exerted by a column of water is directly proportional to the height of the column and the density of the water and is independent of the cross-sectional area of the column.   The pressure thirty feet below the surface of a one inch diameter standpipe is the same as the pressure thirty feet below the surface of a large lake. Example 1: If  the  tank  in  Figure  1  is  filled  with  water  that  has  a  density  of  62.4  lbm/ft³,  calculate  the pressures at depths of 10, 20, and 30 feet. Solution: P r  h  g g_c P 10  feet 62.4^lbm ft³ 10  ft 32.17 ft sec² 32.17 lbm  ft lbf  sec² 624^lbf ft² 1  ft² 144  in² 4.33 lbf in² P 20  feet 62.4^lbm ft³ 20  ft 32.17 ft sec² 32.17 lbm  ft lbf  sec² 1248^lbf ft² 1  ft² 144  in² 8.67 lbf in² Rev. 0 Page 5 HT-03